Add a stronger validation when attempting to configure a builder from a missing package

[natebosch]

Around here: https://github.com/dart-lang/build/blob/593931a940dfa6236c50d034e51bc00ff97f5ac0/build_runner_core/lib/src/validation/config_validation.dart#L26

We have a validation for the full package_name:builder_name key, but if the package doesn't exist at all we could give a more clear error.